Photocatalytic metal‐free oxidation of alcohols with molecular oxygen in supercritical CO2 medium

2-Fluoroanthraquinone has been identified as a non-toxic and accessible organic photocatalyst for green oxidation of alcohols to carbonyls in supercritical (sc) CO₂. Tuning of scCO₂ medium near the critical point allowed attaining high conversions (90–99 %). Molecular oxygen and air were used as safe and atom efficient oxidants. A plausible Hydrogen Atom Transfer (HAT) mechanism of the catalytic photooxidation process has been proposed.

Abstract

Near-UV-light-induced oxidative conversion of cyclic and linear alcohols into corresponding carbonyl compounds was achieved in the supercritical CO₂ medium under the action of molecular oxygen in the presence of 2-fluoroanthraquinone (1 mol %), a simple and available metal-free photocatalyst. A thorough examination of the impact of various process parameters on the reaction outcome allowed to identify a narrow density area at ~0.3 g/cm³ in the vicinity of the medium critical point and optimal reaction conditions (~45 °C, ~8.4 MPa) where up to 99 % conversions and 65–93 % yields of the oxidation products could be attained. Furthermore, it has been shown that compressed atmospheric air, a far cheaper and safer oxidizer than pure oxygen, can be applied to the reaction. Based on the experimental data, a plausible mechanism of the photocatalytic process has been proposed comprising photocatalyst excitation with near-UV light followed by the hydrogen atom transfer (HAT) as the key stages.

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